Quality of the coffee drink depends greatly on the type of cherry processed because while ripe coffee cherries allow obtaining high quality per cup, green coffee cherries produce astringent and unpleasant flavours deteriorating quality of the drink. Therefore, it is clear to be convenient to carry out processing of ripe coffee cherries and green coffee cherries separately.
Despite the above, use of machines during harvesting of coffee cherries, as well as non selective manual harvesting of the coffee crop has evidenced an increase in the percentage of green cherries mixed with harvested ripe cherries, which unfortunately implies the need of carrying out a separation process of green and ripe coffee cherries before the processing stage, which is not simple considering, for example, that all coffee cherries have the same density, and therefore, their separation can not be made by flotation.
Different mechanisms exist at present for separating green from ripe coffee cherries. For example, electronic machines are known selecting the cherry from its colour, technology based on the use of lens and sensors for detecting bean by bean differences in colour reflection. Despite said method does not require water and power consumption is low, its application is very limited due to the high cost of equipment and low processing capacity.
Other type of mechanisms for separating mixtures of green and ripe coffee cherries known at present are directly related to the depulping process. Thus, for example, green and ripe coffee cherry separating machines have been designed which functioning principle lies on the initial compression of the mixture on walls of a perforated metallic basket, so that exerted pressure allows only breaking of pulp of ripe cherries and their subsequent passing (pulp and coffee beans in separate) through perforations of the metallic basket. In other hand, green whole cherry does not pass through perforations of the basket because of its size and hardness, and then it leaves by a side thereof.
However, performance of said equipment represents certain type of problems, among which, there are: 1) use of lot of water is required to keep perforations of the basket clear and also to regulate power consumption; 2) subsequent stage is required to separate the pulp from the beans of the ripe cherry, which is made usually by methods such as flotation or by metallic screens, and therefore, they also require high water and power consumption; 3) water contacted with the pulp and coffee mucilage becomes contaminated and its recovery implies high treatment costs; and 4) non depulped ripe cherries are mixed with green cherries, and smaller green cherries pass through perforations of the metallic basket together with the pulp and depulped ripe beans, then, subsequent processes are necessary for their recovery.
In other hand, patent document U.S. Pat. No. 3,139,919 teaches a machine designed to depulp, wash and classify coffee, involving a disk depulping machine, a coffee friction washer machine and a screen classifying coffee bean by size. This document emphasizes that said machine allows selectively depulping ripe cherries as a result of the position of disks in the depulping machine and of differences in sizes existing between ripe and green coffee cherries. However, said performance principle does not consider the existence in the crop of small ripe and green cherries, as well as the existence of big ripe and green cherries, then said system is evidently not effective to carry out an efficient selective depulping process. Additionally, depulping machine described in document U.S. Pat. No. 3,139,919 requires lot of water for its performance, and this technology is currently obsolete for evident reasons.
On the other side, at present there are known vertical depulping machines using rigid aprons, as well as horizontal cylinder depulping machines using high hardness and low elasticity rubber bottomed aprons, working together with circular screens as mechanism for separating green and ripe coffee cherries. However, due to the need of avoiding depulping of green cherries, structure and position of aprons in said machines must be calculated so that depth of the channel is higher than the size of the green cherry, which evidently leads to a high process inefficiency because small ripe cherries are also able to pass whole together with green cherries, forcing an additional process for recovering said ripe cherries.
In turn, screens are machines in form of flat tray which bottom is a slotted sheet moving laterally by cycles by and eccentric action, which causes materials of smaller size than the slots are able to pass through them, whereas remaining material (that is, of higher size) to be finally removed in separate. Unfortunately, said machines are characterized by their low process capacity, their instability and by the excessive noise they generate.
On the other side, circular screens are rotating cylinders of great size attached to a rotating shaft by metallic arms, and which are partially covered by a slotted sheet or by bars spaced each other. Said arrangement allow rotating movement of the screen to induce passing of materials of smaller size through the slots or spaces, and confining inside greater material, thereby achieving separation of materials based on their size. However, said apparatuses are not suitable either to carry out separation processes due to their excessive size and high cost.
Finally, it is very important to emphasize that the process known currently for depulping ripe coffee cherries in horizontal, vertical or disk depulping machines, is composed by four (4) fundamental stages:    1. Dosing the amount of coffee to be depulped by a rotating mechanical element;    2. Entering of cherries to the depulping zone which is formed by the dentate jacket or grate and the apron;    3. Squeezing coffee cherries between the rigid channel-shaped bottom of the apron and the dentate jacket or grate until pressure exerted surpasses breaking resistance of the pulp and make two coffee beans (formed by endosperm, tegument, endocarp and pectin layer) separate; and    4. Removing pulps from the depulping zone which are hooked in teeth of the dentate jacket or grate, and leaving of depulped beans by a different opening located at the end of the apron channel.
Form the above, it is clear that said depulping mechanism implies squeezing of coffee cherries between two rigid walls (bottom of the apron and dentate jacket or grate), then, pressure that green cherries thus processed are subjected to, is able not only to generate their depulping (favouring their mixture with ripe beans also depulped), but also to imply generation of mechanical damage in the bean. Thus, for example, it is known that depulping process made between rigid and abrasive surfaces produces a breaking of an amount equivalent to 5% and 15% parchment of green beans, and between 0.2% and 1% depulped ripe beans.